SEMICONDUCTOR GROWTH TECHNIQUES Introduction to growth techniques (bulk, epitaxy) Basic concepts in epitaxy (MBE, MOCVD)
Growth Processes Bulk techniques (massive semiconductors, wafers): Si, compounds semiconductors. Epitaxy (higher cost of the growth process): high control of interfaces thin films, quantum confined systems.
Growth techniques for bulk semiconductors: Crystal pulling (Czochralski method) The CZ technique consists of dipping an oriented seed into the molten charge. The solid-liquid equilibrium is established and the seed is pull out to obtain a large crystal. The melt will freeze following the cristallographic orientation of the seed.
Growth techniques for bulk semiconductors Horizonal/Vertical Bridgman and Vertical Gradient Freeze The method consists of a boat which is translated across a temperature gradient in order to allow the molten charge contained in the boat to solidify starting from an oriented seed. An excess of Group V (As, P) is necessary to control the melt composition.
EPITAXY ε π ι + sopra HETEROEPITAXY τ α ξ ι ς ordine, ordinamento disposizione delle schiere Epitaxial growth of a layer with a chemical composition and sometimes structural parameters different from those of the substrate. The heteroepitaxy crucial problems are related to lattice mismatch. ε Growth process of a solid film on a crystalline substrate in which the atoms of the growing film mimic the arrangement of the atoms of the substrate pseudomorphic overlayer (a<a, os o h<h) ε misfit strain str asi aoi aoi aoi fi =, εi =, i = x, a a oi oi y substrate a s
Why Epitaxy? Sizes < 10nm υ structure and composition control with accuracy better than the single atomic monolayer (~0.3nm) Semiconductor growth techniques that allow this control are called epitaxial techniques. Growth takes place on planar, single-crystal substrates, atomic layer by atomic layer.
Growth modes FM: Frank-van der Merwe (2D) mode layer by layer growth SK: Stranski- Krastanov (2D+3D) mode layer plus island growth VW: Volmer-Weber (3D) mode island growth Θ < 1 ML 1 ML<< Θ 2 ML Θ > 2 ML The interaction between substrate and film atoms is stronger than that between neigh-boring layer atoms intermediate case The interaction between neighboring layer atoms exceeds the overlayersubstrate interaction
Examples: Frank-van der Merwe growth TEM micrograph of the active region of a lattice-matched AlInAs/GaInAs QCL grown by MBE Cho et al., J. Cryst. Growth 227-228, 1 (2001) Layer-by-layer growth (Frank - van der Merwe) is the most used epitaxial process in semiconductor device production. It is most often realized for lattice matched combinations of semiconductor materials with high interfacial bond energies (i.e., Al x Ga 1- xas/gaas).
Examples: Stranski-Krastanov growth Stranski-Krastanov - grown islands can be overgrown by the same barrier material as the substrate, to form buried quantum dots, completely surrounded by a larger band gap barrier material. AFM image of uncapped InAs/GaAs quantum dots formed just afted the critical thickness on a wetting layer showing monolayer-high 2D islands. The sample is MBE-grown at TASC.
Steps for Deposition to Occur Every film regardless of deposition technique (PVD, CVD, sputtering, thermally grown ) follows the same basic steps to incorporate molecules into the film. Absorption/desorption of gas molecule into the film Physisorption Chemisorption Surface diffusion Nucleation of a critical seed for film growth Development of film morphology over time b All processes must overcome characteristic activation energies E i, with rates r i exp(e i /kt), depending on atomic details of the process Arrhenius-type exponential laws a c a d e f
MBE Technique (MBE)
Molecular Beam Epitaxy (MBE) Ultra-High-Vacuum (UHV)-based technique for producing high quality epitaxial structures with monolayer (ML) control. Introduced in the early 1970s as a tool for growing high-purity semiconductor films. One of the most widely used techniques for producing epitaxial layers of metals, insulators and superconductors. Research and industrial production applications (Al-containing, high speed devices). Simple principle: atoms or clusters of atoms, produced by heating up a solid source, migrating in UHV onto a hot substrate surface, where they can diffuse and eventually incorporate. Despite the conceptual simplicity, a great technological effort is required to produce systems that yield the desired quality in terms of material purity, uniformity and interface control.
Three-phases model 1. Gas phase (molecular beam generation + vapor mixing zones): complete lack of order, no homogeneous reactions (ballistic transport). 2. Crystalline phase (substrate, epilayer): complete short- and long-range order. 3. Near-surface transition layer (substrate crystallization zone): area where all processes leading to epitaxy occur (heterogeneous reactions on hot surface). Layer geometry and processes strongly dependent on growth conditions. heating block substrate substrate crystallization zone vapor elements mixing zone molecular beam generation zone
Research and production MBE systems R & D Riber Compact21 system: Vertical reactor Up to 1X3 wafer 6 to 11 source ports Production Riber MBE6000 system: Up to 4X8 wafers (MBE7000 model) 10 large capacity source ports Fully motorized wafer handling and transfer
Schematics of an MBE system
Pumping system Minimization of impurities: R ~ 1ML/sec, n ~ 10 22 at cm -3, p ~ 10-6 Torr for n imp < 10 15 cm -3 p < 10-13 Torr In practice: p ~ 10-11 10-12 Torr, mostly H 2 Used pumps: ion, cryo, Ti-sublimation.
Effusion cells Thermocouple Connector Crucible Heat Shielding Filament Thermocouple Power Connector Head AssemblyMounting Flange and Supports Principle Features of operation an effusion of the cell Knudsen cell. The 6 most 10 cell common in a type source of MBE flange source is the effusion cell. Sources of this type are sometimes called Knudsen cells, or K-cells, in reference to the evaporation sources used by Knudsen in his studies of Co-focused onto substrate flux uniformity molecular effusion. However, a true Knudsen cell has a small diameter orifice (<1mm) to maintain high pressure Flux stability within the crucible. <1% / day With certain ΔT exceptions, < 1ºC @ this T ~ practice 1000 ºC is undesirable in MBE because it limits deposition Temperature rates. Conventional regulation MBE by effusion high-precision cells are usually PID fit regulators with a removable, open-faced crucible having Minimization a large exit aperture. of flux The drift crucible as material and source is material depleted are heated choice by radiation of geometry from a resistively heated filament. A thermocouple is used to allow closed loop feedback control.
Crucibles Cylindrical Crucible + Good charge capacity + Excellent long term flux stability - Uniformity decreases as charge depletes - Large shutter flux transients possible Conical Crucible - Reduced charge capacity - Poor long term flux stability + Excellent uniformity - Large shutter flux transients possible SUMO Crucible + Excellent charge capacity + Excellent long term flux stability + Excellent uniformity + Minimal shutter-related flux transients
Cell shutters Function: flux triggering Materials: Ta Mo Mechanical or pneumatic actuators Operation (~50ms) much faster than ML deposition time (~1s) Designed for more than 1 million cycles Not outgassing from cell heating Minimization of heat shield no flux transients Computer control for reproducibility
Substrate manipulator Continuous azimuthal rotation uniformity Heater behind sample (Ta, W, C): temperature uniformity, small outgassing Beam flux monitor (BFM) opposite to sample for flux calibration Temperatures up to >1000C
Wafer holders Mo- or Ta- made holders Bonding: In (Ga), or In-free (clamped) Quick and easy transfer Image: In-free, 3-inch sample holder fitting a quarter of a 2- inch wafer
Reflection High Energy Electron Diffraction Cells substrate RHEED // substrate Control of the crystallographic structure of the growing epitaxial surface Pattern for 2D surface: series of // lines Si(001) RHEED patterns sputter-cleaned surface perfect surface high density of step rough surface